Sea surface temperature

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Sea surface temperature
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Outline

• Why map sea surface temperature?
• How to derive surface temperature from satellite
  data
• Where can you get data?
• Learn to interpret a temperature map

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What is sea surface temperature?

• Definition how hot or warm the water at the
  oceans surface is.
• Water is generally warmest near the surface and
  at the tropics, due to heating by the sun.
• Currents carry cold/warm water around
• Vertically mixed water colder
• Satellite measurements give us the temperature of
  the top 1 mm of water
• Ship measurements are made at 1 m depth

Temperature vs depth
Average temperature map
Temperature
From Tomczak and Godfrey, 1994
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Ways to use remotely-sensed surface temperature

• Mapping (what is the distribution of
  temperature?)
• Regional and global vegetation monitoring
• Snow and ice mapping
• Fire fuel mapping
• Continental scale mapping (less than 13,000,000)
• Soil moisture analysis
• Thresholding (where is a defined temperature
  event occurring?)
• Drought warnings
• Forest fire detection
• Volcanology
• Sand storm observation
• Vegetation indices
• Edge detection (where are large gradients in
  temperature?)
• Dynamic oceanography -- our lab exercise!
• Hydrology, geology, land use

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We map sea surface temperature if we want to
know...

• Where water is cold/warm enough for certain
  species
• Where water is going
• Where coastal storms might gain energy from the
  ocean
• Where ocean mixing is happening
• Upwelling
• Horizontal stirring
• If the ocean is warming
• Climate change?
• El Nino

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Example of GIS and remotely-sensed temperature in
coastal management
From A GIS for near real-time use of remote
sensing in fisheries management in the Gulf of
Mexico. Leming et al, 1999.
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Satellites collecting temperature data

• NOAA Polar Operational Environmental Satellites
  -- what well use in the lab.
• NOAA Geostationary Operational Environmental
  Satellites
• NOAA Terra and Aqua
• Tropical Rainfall Monitoring Mission
• Others

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The Advanced Very High Resolution Radiometer
(AVHRR)

• Collecting data since 1978
• Currently 3 in orbit
• 4 images a day, but cant see through cloud
• Whiskbroom scanning pattern
• 1.1 km2 resolution
• 5 spectral channels
• Visible - channels 1, 2
• Infrared - channels 3, 4, 5
• Calibration is ongoing

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The AVHRR viewing the Earth
Satellite in polar orbit
Sun
Satellite zenith angle
Scan angle
Earth
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How to get sea surface temperature

• Convert measured radiation to brightness
  temperature (temperature of a blackbody emitting
  that radiance)
• We know the emissivity of the sea surface (ratio
  of radiation it emits to that of a blackbody at
  the same temperature and wavelength)
• This gives us sea surface temperature (algorithm)
• Calibrate against ocean buoy temperatures

Radiance measured by satellite
Algorithm
Temperature
Calibration against buoy measurements
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Community algorithm development

• Many algorithms exist for converting radiance to
  sea surface temperature. They differ by
• Number of channels used
• Whether they apply to night vs day
• Which NOAA satellite they apply to
• Whether they need a first guess of the
  temperature
• We will use the Multi-channel SST algorithm
  developed by NOAA.
• Daytime
• Split Window 2 channels used
• Commonly used, updated.

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The algorithm

• MCSST Day Split Window Algorithm
• Developed by NOAA
• Continually calibrated by NOAA

• T4 brightness temperature in channel 4
• T5 brightness temperature in channel 5
• satellite zenith angle
• a, b, c, d coefficients that match the
  algorithm temperature to temperatures measured by
  drifting and moored buoy data

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Example of AVHRR/buoy temperature matchup
New England
From NOAA CoastWatch Northeast Node
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Visible channels

• Cloud screening uses visible channel data
• Visible channels are calibrated to give the
  reflectance factor
• Ratio of the radiance of the actual surface to
  the radiance of an ideal Lambertian surface
  illuminated and viewed in the same manner as the
  surface of interest

Source http//snrs.unl.edu/agmet/908/brdf_definit
ion.htm
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What contaminates surface temperature
measurements?
Sun
Sun glint
Intervening atmosphere
Cloud top temperature
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Corrections

• Temperature estimates deteriorate as the
  satellite looks farther from nadir.
• Viewing angle
  cutoff
• In clouds, measure cloud top temperature, not sea
  surface temperature.
• Compared the the sea surface, clouds are
• Patchy Uniformity test
• Bright Albedo cutoff
• Cold Temperature cutoff
• Different temperature in different channels due
  to water vapor absorption
• Difference cutoff

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Example Masking out clouds over western Europe
Source http//landlub.ucsd.edu/projects/asmc/asmc
.html
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An AVHRR image of the Washington Coast

• Date Sept 1, 1999, about 3pm local time
• Cloudy offshore, clear towards the coast

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What Ive already done in ENVI

• Calibrated the raw data
• Visible channels reflectance, where 100
  highest possible radiance in the band
• Thermal channels brightness temperature in
  Kelvin
• Calculated sea surface temperature using the
  MCSST algorithm (Celsius)
• Georegistered the image
• Projection UTM zone 10 for Washington coast
• Chose all 51 geolocation (warp) points in the X
  direction
• Chose at least (number of image lines)/10 in the
  Y direction
• Polynomial warp of data onto projection
• Cropped the image to the Washington Coast region

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How youll use the image

• Mask out the clouds
• Choose a threshold for Band 2 reflectance
• AVHRR cant see through clouds, so it records the
  (cold) cloud top temperatures, not the surface
  temperature
• Locate regions where upwelling is occurring
• Thresholding
• Upwelling is vertical mixing of the water,
  bringing colder water and nutrients to the sea
  surface. The result is plankton blooms and high
  chlorophyll production.
• Locate ocean fronts
• Edge detection

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Where you can get data

• Processed SST data
• NOAA CoastWatch
• http//coastwatch.noaa.gov/
• West coast regional node
• Registration required, free
• Binary data (ArcView, NetCdf, HDF, or their own
  file format)
• Raw SST data for you to process yourself
• Satellite Active Archive
• http//www.saa.noaa.gov/
• Registration required, free
• Binary data must be read in to your software
  package
• File format changed with satellites after N14!

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Key points

• Remotely-sensed surface temperature maps have
  many uses
• Geophysical research (oceanography)
• Resources monitoring
• The Advanced Very High Resolution Radiometers
  measurements can be converted into brightness
  temperature and reflectance factors.
• Sea surface temperature is derived from
  brightness temperature in the infrared bands,
  using a community-developed algorithm.
• Reflectance factor is derived from visible band
  radiance, and is used to mask out clouds.
• To identify geophysical features, we use
  thresholding and edge detection.
• Data are freely available on the web.

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References/ to learn more
Sea surface temperature 1. M. Tomczak and J. S.
Godfrey Regional Oceanography an Introduction.
Pergamon, New York, 1994. 2. What is sea
surface temperature? http//www.csc.noaa.gov/crs/
definitions/SST.html Using AVHRR to get sea
surface temperature 1. May et al, Operational
processing of satellite sea surface temperature
retrievals at the Naval Oceanographic Office.
Bull. of the Amer. Met.l Soc. 79 (3),
397-407. (http//ams.allenpress.com/amsonline/?req
uestget-tocissn1520-0477volume079issue03)
2. Equations Used in ENVI to Calculate Sea
Surface Temperature from AVHRR Data
(http//www.rsinc.com/services/output.cfm?tip_id2
798) ENVI processing of AVHRR images 1. Tips
for georeferencing AVHRR images in ENVI
(http//www.rsinc.com/services/output.cfm?tip_id1
632)